Catalytic cracking units are generally constituted by a reaction zone in which the catalyst is brought into contact with a hydrocarbon feed in a reactor which is generally in the form of an elongated tube, then at least partially separated from the hydrocarbons in one or more separation stages, the hydrocarbons, accompanied by as small a quantity as possible of catalyst, leaving the reaction zone to rejoin the hydrocarbon fractionation section. The catalyst from the different separation stages is brought into contact with a gas which is different from the hydrocarbons, such as nitrogen or steam, to encourage desorption of the hydrocarbons entrained in the pores of the catalyst, this phase generally being known as stripping. The catalyst is then evacuated to a regeneration zone where the coke formed during the reaction in the tube reaction and hydrocarbons which have not yet been desorbed during the stripping stage are burned in an oxidising medium.
In order to obtain good selectivities for upgradable products in the reaction zone of the catalytic cracking unit, it is necessary:
to rapidly evacuate the gaseous products produced in the contact zone between the hydrocarbons and catalyst after the first separation stage to avoid thermal degradation of the intermediate products of the cracking reactions which generally have the highest added values; PA1 to limit the entrainment of hydrocarbons with the catalyst, and thus to produce effective catalyst stripping.
A number of ways exist for carrying out these operations of separation of desorption and the literature is fall of devices developed for catalytic cracking and which are more or less effective for such different operations. And while it is relatively simple to carry out rapid separation or effective stripping, it is difficult to carry out rapid separation and effective stripping simultaneously.
Thus rapid separation can be effected using cyclones directly connected to an upflow reactor, usually termed a riser in the art, as described in U.S. Pat. No. 5,055,177. In such systems, cyclones connected to the riser are inside a large vessel which generally also encloses a second cyclone stage. The gas separated in the first stage enters the second cyclone stage for more severe separation. The catalyst is directed into the dense phase of a fluidised stripping bed where steam is injected as a counter-current to the catalyst to desorb the hydrocarbons. Such hydrocarbons are then evacuated from the reactor in the diluted phase and introduced into the separation system into the second cyclone stage. The fact that there are two cyclone stages, one connected to the riser carrying out primary separation, the second generally being connected to the outlet for gas from the first stage cyclones, necessitates a very large diameter for the vessel surrounding the two cyclone stages. That vessel is only travelled by the gases desorbed in the stripper, or by the gases entrained by the catalyst in the solid outlets (diplegs) of the first stage. The gases from the stripping section are thus systematically exposed to a long term thermal degradation in the stripper since if the primary cyclone functions correctly, a fairly small quantity of hydrocarbons is entrained in the dipleg of the primary cyclone towards the stripper. The volume of the large vessel being large, and the quantity of hydrocarbons and stripping steam being fairly small, the surface velocity of the gases in the diluted phase of the reactor outside the primary cyclones will not be above a few centimeters per second and the evacuation time for hydrocarbons stripped or entrained in the diplegs with the catalyst will necessarily be of the order of one to a few minutes.
A further disadvantage of that separation system is that it introduces hydrocarbons entrained or adsorbed onto the catalyst in localised fashion into the fluidised stripping bed. Since the fluidised bed is a poor radial mixer but a very good axial mixer, there is an inevitable loss of efficiency in the stripping zone. It would be possible to improve stripping by introducing stripping gases directly into the solid outlet. Nevertheless, this would only be effective if the catalyst flowed slowly in the cyclone outlet in order not to entrain gases, which is not possible to achieve if proper operation of the primary cyclones is to be retained.
It is also possible to bring the hydrocarbons and catalyst into contact in a dropper reactor as described in French patent FR-A-2,753,453. That type of very rapid, homogeneous piston contact is generally characterized by shorter contact times than in riser type apparatus, enabling higher temperatures and higher catalyst circulation rates to be used, and thus encouraging the formation of added value products such as LPG, and in particular olefins and gasoline. More so than with a riser reactor, such conditions necessitate effective separation of the hydrocarbons from a large portion of the catalyst in a short period and with proper integration with the catalyst stripping phase.
Rapid separation can also be carried out in a single induced vortex chamber such as that described in U.S. Pat. No. 5,584,985. That technology, termed a vortex separation system, has the advantage of simultaneously combining separation and stripping.
Solids stick to the wall under a centrifugal effect and flow towards the base of the vessel where they are brought into contact with a stripping gas which is evacuated with the desorbed hydrocarbons towards the top of the separation chamber. Unfortunately, to obtain good separation efficiency with such a system, the chamber size must be limited in order for the centrifugal force exerted on the particles to be sufficiently high. That is incompatible with a low stripping gas rise rate to limit the re-entrainment of particles descending after they have been separated. Combining separation and stripping in the same chamber thus does not enable the two operations to be carried out properly. Either separation is favoured to the detriment of stripping, or stripping is favoured to the detriment of separation, which is not compatible with proper hydrocarbon desorption. Further, and this constitutes a major problem with such technology, the separated catalyst preferentially flows on the wall and thus is not easily brought into contact with the stripping gas which is distributed over the whole cross section of flow of the chamber.
The two examples given above clearly show that it is difficult to carry out separation and stripping in a single chamber, and that rapid separation using known high performance separators such as cyclones necessarily involves deterioration of the performances of the stripper.
One aim of the present invention is to overcome the disadvantages of the prior art. We have thus sought to develop a technique which enables sufficient separation efficiency to be obtained, i.e., over 75%, combined with a desorption apparatus, the assembly being highly compact to enable all of the gas moving from a riser or dropper to the fractionation column associated with the reaction zone to be resident for less than 3 seconds in the separation and stripping zone, to ensure good contact between the catalyst from the separation chambers and the stripping gas, and to evacuate the desorbed hydrocarbons rapidly because of optimal compactness of the equipment.